1. Field of the Invention
The present invention relates to a very high frequency television tuner circuit. More specifically, the present invention relates to an improvement in a circuit configuration at the radio frequency amplifying stage in a VHF television tuner for selection of VHF television channels signals and a UHF/VHF converted television channel signal through mechanical switching selection of inductance coils.
2. Description of the Prior Art
Typically, a very high frequency or VHF television tuner includes a mechanical switching selection scheme of inductance coil means for selection of a broadcasting television signal, a radio frequency amplifier operatively coupled to the inductance coil means for selectively amplifying a desired radio frequency signal, and a local oscillator operatively coupled to the inductance coil means for providing a local oscillation frequency signal, the frequency of which is different by a given frequency difference from the selectively amplified radio frequency signal. The tuner also includes a mixer responsive to the selectively amplified radio frequency signal from the radio frequency amplifier and the local oscillation signal from the local oscillator for providing an intermediate frequency signal, and an intermediate frequency tuned circuit for selectively withdrawing the intermediate frequency signal.
The above described mechanical switching selection scheme of inductance coil means may comprise a so-called switch type structure, i.e. a plurality of sets of coils, each set allotted for one channel, provided in a stationary manner along the periphery around a given axis, and a switching contact arrangement rotatably mounted about the axis so as to be rotated by means of a channel selection shaft.
FIG. 1 is a block diagram of a typical VHF television tuner VT. The tuner VT is shown connected to an antenna VA for receiving a broadcasting VHF television wave and comprises a radio frequency amplifier RFA preceded and succeeded by a preceding stage radio frequency tuned circuit RFT1 and a succeeding stage radio frequency tuned circuit RFT2, respectively, for selectively amplifying the desired radio frequency signal, a local oscillator OSC for providing an oscillation frequency signal, the frequency of which is different by a given frequency difference from the selected or desired radio frequency signal, a mixer MIX responsive to the received radio frequency signal from the succeeding stage radio frequency tuned circuit RFT2 and to the local oscillation frequency signal from the local oscillator OSC for providing an intermediate frequency signal, and an intermediate frequency tuned circuit IFT for selectively withdrawing the intermediate frequency signal, which is shown succeeded by an intermediate frequency amplifier IFA in a television receiver.
The tuner shown in FIG. 1 is also shown connected to a UHF/VHF converter UT which is connected to an antenna UA for receiving a broadcasting UHF television wave. Typically, the UHF/VHF tuner is structured to convert a UHF television signal into a VHF television signal of a predetermined frequency and having the same television information. Accordingly, the VHF television tuner VT is adapted to receive selectively the above described VHF television signal of a predetermined frequency converted from the UHF television signal.
The radio frequency tuned circuits RFT1 and RFT2 each comprise inductance coils L1 and L2, respectively, which are to be selected for each channel by means of a mechanical switching selection means (not shown in FIG. 1), described previously, and each are shunted by capacitors C1 and C2, respectively, to form radio frequency tuned circuits. The oscillator OSC also comprises an inductance coil L3, which is also selected for each channel by means of the mechanical switching selection means (not shown in FIG. 1) and is shunted by a capacitor C3 to form an oscillator tuned circuit OSCT. Although the inductance values of the coils L1, L2 and L3 are changeable by means of the mechanical switching selection means, and hence the frequencies of the tuned circuits RFT1, RFT2and OSCT are accordingly changeable for the purpose of selection of a desired channel, the difference between the frequencies of the radio frequency tuned circuits RFT1 and RFT2 and the oscillator tuned circuit OSCT is kept constant, so that a constant intermediate frequency signal may be obtained from the mixer MIX. Therefore, the intermediate frequency tuned circuit IFT is tuned to the predetermined intermediate frequency.
In implementing a VHF television tuner as shown in FIG. 1 for selection of VHF television channel signals and a UHF/VHF converted television channel signal through mechanical switching selection of inductance coils, it is necessary to selectively connect a few capacitors to the selected radio frequency inductance coils simultaneously with switching selection of the radio frequency inductance coils, as is well known to those skilled in the art. In addition, the tuner should be capable of switching the input terminal to selectively receive a VHF television signal or a UHF/VHF converted television signal. Hence, a conventional VHF television tuner for selection of VHF television channel signals and a UHF/VHF converted television signal through mechanical switching selection of inductance coils comprises a complicated mechanical switching scheme.
A conventional VHF television tuner for selection of VHF television channels and a UHF/VHF converted channel through mechanical switching selection of inductance coils uses one blade for selective connection of each capacitor to be selectively connected to the selected inductance coil between the high and low band channels and also uses one separate blade for selection of the UHF/VHF converted channel; however, this increases the number of sections and the number of blades in the channel selection switch arrangement. This accordingly increases the number of stationary contacts or clips. Thus, the structure of the channel selection switch arrangement for use in the above described VHF television tuner becomes complicated, which makes the cost expensive. In addition, more time is consumed for assemblage of such a VHF television tuner.
An improved VHF television tuner circuit for selection of VHF television channels and a UHF/VHF converted television channel through mechanical switching selection of inductance coil means, which is of interest to the present invention, is described and claimed in the copending U.S. patent application, Ser. No. 780,112, now U.S. Pat. No. 4,112,378 entitled "VHF Television Tuner Circuit", filed by Katsuo Ito et al. and assigned to the same assignee as the present invention. More specifically, the referenced patent application describes a VHF television tuner circuit for selecting a channel from a plurality of VHF television channels including high band, low band, and a UHF/VHF converted television channel, by mechanical selection to form a resonant circuit having electrical characteristics corresponding to the selected channel, and thus to tune to a desired radio frequency signal. The tuner circuit includes a first means for receiving television signals comprising VHF television signals, and producing the VHF television signals as an output thereof; a second means for receiving television signals comprising UHF/VHF converted television signals, and producing the UHF/VHF converted television signals as an output thereof, a channel selection means operator actuable for selecting one of a plurality of channel positions including high band, low band and UHF/VHF converted channel positions corresponding to the selected channel; a plurality of induction coils; a pair of capacitors; and radio frequency amplifying means for selectively amplifying the desired radio frequency signal. The channel selection means includes only a first switch and a second switch; the first switch includes an inhibiting means responsive to selection of the UHF/VHF converted channel position for inhibiting the output of the first receiving means, and means responsive to selection of one of the high band and low band channel positions for connecting corresponding at least one of the plurality of induction coils to the second switch to partially form the resonant circuit. The second switch includes a means responsive to selection of the UHF/VHF converted channel position for connecting the second receiving means to the radio frequency amplifying means, a means responsive to selection of one of the high band channel positions for connecting the pair of capacitors in parallel to corresponding at least one of the plurality of induction coils to completely form the resonant circuit having electrical characteristics corresponding to the selected one of the high band channels. A means is provided which is responsive to selection of one of the low band channel positions for connecting a first one of the pair of capacitors to corresponding at least one of the plurality of induction coils to form a junction therebetween, and for connecting a second one of the pair of capacitors between said junction and said radio frequency amplifying means, whereby to completely form the resonant circuit having electrical characteristics corresponding to the selected one of the low band channels.
According to the invention described and claimed in the referenced patent application, the number of stages of the switching arrangement for the radio frequency tuned circuit is decreased while the electrical characteristic of the tuner circuit is maintained and thus the number of circuit components is decreased which is required for the radio frequency tuned circuit in a VHF television tuner for selection of VHF television channels and a UHF/VHF converted television channel through mechanical switching selection of inductance coil means.
According to a preferred embodiment of the invention of the referenced patent application, the inhibiting means of the first switch comprises means for establishing an open circuit at the first receiving means output. As a result, if and when the UHF/VHF converted channel position is selected, the VHF television signal path is disconnected from the first receiving means. In general, the first and second switching means are provided at the front and rear surfaces of an insulating plate, which causes a stray capacitance therebetween. Therefore, if the VHF television signal transmission path had been connected to the first receiving means when the UHF/VHF converted channel position had been selected, then a cross feed interference of the VHF signal component could have occurred through the stray capacitance. However, according to the above described preferred embodiment of the referenced invention, such cross feed interference is effectively eliminated and the so-called UHF/VHF separation characteristic is advantageously improved, because the VHF television signal path is disconnected from the first receiving means.
Typically, the first switch comprises a plurality of stationary contacts corresponding to the selectable UHF/VHF converted channel position, the high band channel positions and the low band channel positions, and movable contacts positioned in proximity to the stationary contacts for coaction therewith. On the other hand, the first switch includes the inhibiting means responsive to selection of the UHF/VHF converted channel position for inhibiting the output of the first receiving means. Thus, it follows that a VHF television signal transmission path including the first receiving means is positioned in the vicinity of some of the plurality of stationary contacts. However, this causes another stray capacitance between the first receiving means and the adjacent stationary contacts, which also induces a cross feed interference of the VHF signal component through the stray capacitance and further through the inductance coils to the radio frequency amplifier and thus deteriorates the UHF/VHF separation characteristic: In this context, the VHF television tuner circuit of the referenced patent application still includes room for improvement in the UHF/VHF separation characteristic.